A run-on sentence consists of two or more main clauses that are run together without proper punctuation. Sometimes even sentences which are technically correct are easier to read if they are made into shorter sentences. We often speak in run-on sentences, but we make pauses and change our tone so people can understand us. But when we write, no one can hear us, so sometimes we must break our sentences into shorter units so that they do not sound run-on.

Incorrect: The boy showed us his tickets someone
gave them to him.

Correct: The boy showed us his tickets. Someone
gave them to him.

Incorrect: We often speak in run-on sentences,
but we make pauses and change our tone so people can understand us, but when we write, no one can hear us, so sometimes we must break our sentences into shorter units so that they do not sound run-on.
(Technically punctuated OK, but too long to be easily understood. See better sentence structure above.)